The present invention relates to a system and a method for estimating efficiency of an exhaust gas aftertreatment device and, more particularly, to estimating an exhaust gas component storage capacity of a Lean NOx Trap coupled downstream of an internal combustion engine.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream catalytic converter. The LNT stores exhaust components, such as oxygen and NOx, during lean operation. After the LNT is filled, stored exhaust gas components are typically reduced and purged by switching to rich or stoichiometric operation, i.e., by increasing the ratio of fuel to air.
Over time, the ability of the catalyst to store exhaust gas components can decrease due to such factors as sulfur deposits (SOx) from the fuel. Therefore, when the storage capacity is sufficiently reduced, a SOx purge has to be performed. Typically, the catalyst is heated and engine air-fuel ratio is changed to rich for SOx release and reduction. Since SOx purges result in fuel economy penalties, it is desirable not to purge unnecessarily. Thus, in order to maintain adherence to emission standards and to obtain fuel economy benefits of a lean burning engine, it is desirable to monitor the efficiency of the LNT.
One method of determining the efficiency of the catalyst is by inferring it from the amount of fuel required to perform a NOx purge as described in U.S. Pat. No. 5,713,199. In particular, the method involves saturating the NOx absorbent with NOx up to a predetermined amount (based on an engine model), and allegedly calculating the amount of NOx purge fuel as the difference between total fuel required to purge the NOx absorbent of stored NOx and oxygen, and oxygen purge fuel (determined separately). The amount of NOx purge fuel is then correlated to the device""s efficiency.
The inventors herein have recognized a disadvantage with this approach. Namely, the prior art method uses an engine model to determine the amount of NOx required to saturate the NOx storage device to capacity. However, different operating conditions, such as high or low humidity, may cause this amount to vary significantly. For example, at high humidity levels NOx generation rate is lower than that at low humidity. Therefore, the prior art method may indicate that the LNT is filled to capacity when that may not be the case. Since the fill level of the LNT may not be accurate, the calculated amount of purge fuel required to release and reduce stored NOx may not be accurate either. As a result, it may not be possible to precisely monitor the LNT efficiency.
In solving the above problem, a system and a method are provided for monitoring the efficiency of an exhaust gas aftertreatment device by calculating the amount of purge fuel required to release the exhaust gas components stored in the device during lean operation.
In carrying out the above solution, features and advantages of the present invention, a system and a method for monitoring en efficiency of an exhaust gas aftertreatment device disposed in an exhaust passage of an internal combustion engine, the device having a sensor coupled downstream of it, include operating the engine at an air-fuel mixture lean of stoichiometry; indicating that the device has stored more than a preselected quantity of an exhaust gas component based on an output of the sensor; in response to said indication, switching engine operation to an air-fuel ratio rich of stoichiometry to release said stored exhaust gas components from the device; and calculating an amount of said stored exhaust gas components stored in the device based on an amount of fuel required to complete said release.
The present invention provides a number of advantages. For example, the present invention is much more precise at estimating the efficiency of the LNT since it allows filling the LNT with oxidants to a known level by monitoring the output of the sensor coupled downstream of the LNT, as opposed to the prior art method of assuming the fill level of the LNT based on an engine model. Also, the estimate is made more accurate by the fact that less error is introduced by calculating the fuel required to purge all stored oxidants from the LNT as opposed to the prior art method of calculating NOx and oxygen purge fuel separately. The improved precision will prevent unnecessary purges allowing for more lean running time, and increased fuel economy.
Yet another advantage of the present invention is that monitoring, rather than assuming, the state of the LNT prevents overfilling it with oxidants and possibly releasing some in the atmosphere. Therefore, better emission control is achieved.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.